1. Field of the Invention
The present invention relates to a process for producing oxygen by means of a pressure swing adsorption separation, and more specifically, relates to a process for producing oxygen, having a purity of approximately 90%, from a gas mixture principally containing oxygen and nitrogen, such as air, by means of a pressure swing adsorption separation using an adsorbent capable of selective adsorption of nitrogen.
2. Description of Related Art
As processes for producing an oxygen enriched product by treating a gas mixture principally containing oxygen and nitrogen, such as air, processes for producing oxygen by means of pressure swing adsorption systems (hereinafter referred to as "oxygen PSA process") are widely employed. An oxygen PSA process is generally carried out by using an apparatus which is provided with a plurality of adsorption columns which are packed with zeolite as an adsorbent, which is capable of selective adsorption of nitrogen; such an oxygen PSA process is basically designed to continuously produce an oxygen enriched product by alternately repeating within each adsorption column an adsorption step, in which the operation is carried out at a relatively high pressure, and a regeneration step, in which the operation is carried out at a relatively low pressure.
Such an oxygen PSA apparatus enriches an oxygen product and separates it from air, making use of the highly selective adsorptivity of zeolite for nitrogen; however, since the adsorptivity of zeolite for oxygen is approximately the same as that for argon, the separated and enriched oxygen product contains argon, and has a concentration of approximately 95% at maximum.
On the other hand, with regard to the requirements of the users of this oxygen, in the case when oxygen is used for cutting a metal, oxygen having a concentration of not more than 99.5% causes problems in cutting speed and at the cut surface of materials; the Pharmaceutical Affairs Law in Japan specifies that oxygen concentration is required to be at least 99.5% for oxygen for medical use at hospitals or the like. However, since an oxygen concentration of not more than 95% is sufficient for steel manufacture or the like using an electric furnace, and since the oxygen concentration of about 90% is sufficient for most other applications for oxygen, the applicable range of the oxygen PSA process is very wide. Accordingly, for users for whom the oxygen concentration of about 90% is sufficient and who consume a large amount of oxygen, various improvements have been carried out in the PSA process in order to obtain oxygen at a lower cost.
In order to improve the performance of the oxygen PSA process, attention is directed to two points, namely: raising productivity of oxygen per unit amount of adsorbent to be used, so as to reduce the size of the apparatus; and enhancing product oxygen recovery so as to reduce the unit power consumption.
As described in the above, an oxygen PSA process comprises an adsorption step and a regeneration step as basic steps; in order to enhance oxygen recovery, a pressure recovery step (pressure equalization step), a re-pressurization step, or the like are usually added to the basic steps. Furthermore, there are some cases in which in place of the pressure recovery step, a cocurrent depressurization step is employed so as to utilize an enriched oxygen remaining within the adsorption column as a product or a purging gas; and there are other cases, in order to raise the productivity of oxygen per unit amount of adsorbent, in which a purging (vacuum purge) operation is carried out in the regeneration step with a part of a product gas so as to promote desorption of nitrogen from an adsorbent. This purging operation is a process to reduce the partial pressure of more strongly adsorbable component in the gas phase, thereby promoting the desorption of nitrogen, by way of supplying a part of product gas through an end of a product outlet at the stage when the pressure inside the adsorption column is lowered by evacuation; it is irrelevant whether the purging operation is employed in a regeneration process carried out at normal pressure or in a vacuum.
As an example of a conventional process carried out in order to improve the performance of an oxygen PSA process, Japanese Patent Application, First Publication (Kokai), Sho 63-144104, discloses a process in which a pressure equalization step is employed by which gases are recovered simultaneously from both an upper section (product gas outlet section) and a lower section (feed gas inlet section) of each of two adsorption columns which are connected (upper-lower simultaneous pressure equalization). When this is the case, a great amount of gas can be recovered; however, with regard to the adsorption column which receives the gas, a gas in which oxygen is relatively concentrated is collected within the upper section of the column, and air or a gas which has a nitrogen content higher to some degree than air is collected within the lower section of the column. Therefore, according to this process, the productivity of oxygen per unit amount of adsorbent is comparatively low, although the product recovery is high.
In addition, according to the disclosure in Japanese Patent Application, First Publication (Kokai), Sho 63-144103, in a pressure equalization step, gases are recovered simultaneously from both an upper section and a lower section of each of two adsorption columns which are connected in a similar manner to the above; at this time, the lower line regulates the recovery from the lower section of the column by way of evacuating a part of the gas which is recovered from the lower section by using a vacuum evacuation line. According to this process, problems arise in that the gas recovery is comparatively low in comparison with the above process, and thus the product recovery is not very high, as well as in that since the pressure increase due to the recovery within the column of the receiving side is low, the necessary amount of oxygen for product repressurization in the subsequent pressurization step becomes larger, and the adsorption pressure of the column which is producing oxygen in an adsorption step is reduced.
That is to say, with regard to an oxygen PSA process, since to maintain the product recovery at a high level and at the same time to enhance the productivity of oxygen per unit amount of adsorbent were mutually exclusive requirements, such a process as satisfies both has not been developed.